1. Field of the Invention
The present invention relates to a heat sensitive recording sheet having excellent color forming properties and high stability of the developed image against oily substances such as hair oil, sweat, etc.
2. Prior Art
A heat sensitive recording sheet that utilizes a thermal color-forming reaction occurring between colorless or pale-colored chromogenic dyestuff and phenolic material, or organic acid is disclosed, for example, in Japanese Patent Publication No. 4,160/1968 and No. 14,039/1970 and in Japanese Laid-Open Patent Application No. 27,736/1973 and is now in wide practical use.
In general, a heat sensitive recording sheet is produced by applying to the sheet surface a coating which is prepared by individually grinding and dispersing colorless chromogenic dyestuff and color-developing material, such as phenolic substance into fine particles, mixing the resultant dispersions with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating instantly undergoes a chemical reaction which forms a color. In this case, various bright colors can be advantageously formed depending upon the selection of specific colorless chromogenic dyestuff.
These heat sensitive recording sheets have found a wide range of applications including medical or industrial measurement recording instruments, terminal printers for computers and information communication systems, facsimile equipment, printers for electronic calculators, automatic ticket vending machines and so on.
On the other hand, heat sensitive recording sheets come continuously into contact with human hands. At this time, oily substances such as hair oil, sweat, etc., adhering to the hands and fingers of the operators, often transfers to and contaminates the heat sensitive recording sheet. Generally, heat sensitive recording papers have not had sufficient stability against the oily substances so that there is a phenomenon that the color image density of contaminated parts is reduced or disappears and in that contamination of the background causes discoloration. Such a lack of stability against the oily substances is disadvantageous to the role and function of information recording paper.
Ideally, it is required that a heat sensitive recording paper develop a clear image, exhibit no recording problems due to the adhesion of accumulated residues onto the thermal head, sticking, etc., and has improved quality such as less decline in brightness with elapsed time. In addition, it is required that the heat sensitive paper have excellent stability of the recorded image and background against the oily substances such as hair oil, sweat, etc.. The reason why contamination due to hair oil, sweat, etc. causes the reduction of density or the disappearance of a developed image and the deterioration of the background is unknown, but it is assumed that such an oily substance is contained in hair oil, sweat, etc. will instabilize the reaction product of the chromogenic dyestuff and organic color-developing agent; and in the case of the background, the oily substance will instabilize the color-forming layer composed of chromogenic dyestuff and organic color-developing agent.